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 Post subject: Chassis Earthing
PostPosted: 18 Mar 2015, 00:29 
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Joined: 17 Nov 2014, 01:48
Posts: 9
Location: Melbourne, Australia
I am building a tube audio amp. I am not sure what is the best method to do the grounding. I have taken the mains earth to the chassis to comply with Australian standards. I have also fitted a bus bar for earthing all the components, so that they go back to a central point. My question is this: Should I also ground the "earth bus bar" to the chassis or leave it isolated from the mains earth?

Thanks guys, John


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 Post subject: Re: Chassis Earthing
PostPosted: 18 Mar 2015, 02:34 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Seperate the signal ground from earth with a pair of front-to-back diodes hefty enough to handle the current if all goes wrong (like 6A8) and bypass them with a 100R, 2W non-inductive resistor.

Cheers!


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 Post subject: Re: Chassis Earthing
PostPosted: 18 Mar 2015, 14:36 
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Joined: 04 Jun 2008, 20:59
Posts: 3844
Location: Arizona, USA
You can also use a resistor and parallel capacitor. The capacitor needs to be a type X2 (ones that are rated for AC mains use) and the resistor ought to be at least 1 watt. Values are typically 120-150 ohms and 0.1 to 0.22 uF (voltage ratings are generally standard at 277RMS). I personally prefer the RC combo as it seems to reduce the noise floor in sensitive gear better but either method will work.

Good listening
Bruce

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 Post subject: Re: Chassis Earthing
PostPosted: 18 Mar 2015, 16:40 
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Joined: 17 Nov 2014, 01:48
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Location: Melbourne, Australia
OK, Thanks guys for your help.


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 Post subject: Re: Chassis Earthing
PostPosted: 06 Sep 2015, 10:13 
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Joined: 02 Sep 2008, 14:34
Posts: 527
Location: Rochester, NY
I have seen Pete Millett use the following:

Quote:
I isolated the AC safety ground, which is connected to the AC plug ground and the chassis, from signal ground using a "ground breaker" circuit. This is an arrangement of series / anti-parallel diodes that presents a high impedance unless there is a voltage of more than two diode drops (about 1.4V) across them. These diodes are large - they must be sized such that enough current can flow through them to cause the fuses to blow if there were to be a short between signal ground and the AC line.


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 Post subject: Re: Chassis Earthing
PostPosted: 06 Sep 2015, 15:39 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
I always thought you filled the chassis with sand. :eek:

Good listening
Bruce

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 Post subject: Re: Chassis Earthing
PostPosted: 06 Sep 2015, 20:25 
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Basically the same, Allan... just sets the voltage at 1.4-2V instead of 0.7-1.1. A handy thing if your ground noise peaks Vp-p are high.

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: Chassis Earthing
PostPosted: 16 Apr 2016, 05:17 
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Joined: 01 Jun 2013, 09:05
Posts: 677
gofar99 wrote:
I always thought you filled the chassis with sand. :eek:

Yes but does the diode type make a difference on the ground breaker :confused: should it be soft recovery or a schottky.
Should the capacitor be non inductive.. :eek:

Just a joke erm. :D

On a more serious note
The ground of the supply should be earthed so any faults will clear. The ground breaker isn't always necessary I would try it without first (straight to earth) and use it if necessary.
I use different methods of grounding depends what mood I'm in at the time, however I usually star ground the left and right channel then join them together at a common star ground this means I can isolate the grounding independently. The main thing is that under fault conditions no difference in potential appears between any metalwork or between an audio connector and the chassis.
You must never remove the chassis earth to stop hum.

Worst thing I have seen is someone rely on the audio connectors to ground the chassis and when they unplugged the interconnects a voltage appeared between the hand holding the connector and the other hand holding the chassis. The guy was lucky to survive.

Regards
M. Gregg

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